There are many applications where it is desirable or necessary to automatically provide an indication to an operator of the liquid level in a reservoir without the operator of the liquid level in a reservoir without the operator personnally observing it, or to automatically trigger some operation or function at a predetermined liquid level. By way of example, but not by way of limitation, one such application is in the automatic monitoring of the oil level in the crankcase of an automobile in order to warn the driver of a low oil level. However, although automatic liquid level sensors have been known in various fields for a considerable time, and although several different approaches have been taken in this field, the prior art devices have had shortcomings with respect to meeting the demands of this and other applications. More particularly, with regard to the crankcase of an automobile, the oil itself is hot, is electrically nonconductive, and is subject to movement back and forth in the crankcase due to stop-and-go driving, turning and the like. Also, while the cost of such a sensor should be low and its operation simple, the sensor must be accurate and reliable and should be easily replaced if it should fail for any reason. Further, because of the desirability and prevalent use of electronic controls, its output should be readily compatible with electronic signal processing.
One approach to level sensing utilized multiple electrodes placed in the reservoir at a predetermined level or levels with an electrical voltage potential maintained across the electrodes. When a conductive fluid immersed two or more of the electrodes, an electric current would flow through the conductive liquid between the electrodes and was sensed or otherwise used to activate a switch to provide an indication to an operator, or activate or deactivate additional equipment. In general these devices proved adequate in some situations, but unsuitable for sensing the level of a nonconductive liquid such as oil. Further, they were not readily adaptable to providing a variable output proportional to the level of the liquid.
Other approaches used pressure sensitive switches and/or fluidics. These approaches, while having usefulness in certain applications, required a fluid pressure source, usually many parts, and were not readily adaptable to providing a variable output proportional to the level of liquid in the reservoir. Further, these systems were susceptible to leaks, blockage and other failures which could not be tolerated in many instances.
Still another approach to level sensing used a "float" in the reservoir. A buoyant device would float on the surface of the liquid and rise and fall in response to the level of the liquid in the reservoir. The float was attached for example to a mechanical valve or a switch which responded to the position of the float to provide an indication of the liquid level or to activate some further device. Although these prior art float devices proved adequate in many applications, they were generally complex, having multiple moving parts and mechanical linkages, and they were not suitable for many applications simply due to their particular construction or operating characteristics.
Thus there is today a need for a liquid level sensor compatible with both nonconductive and conductive fluids and which can be used in hostile environments such as, for example, in the hot crankcase of an automobile. There is also a need for a reliable level sensor which is adaptable to automatic electronic sensing without complex mechanical parts and interfacing, Which can easily be replaced and whose output can be filtered electronically so as not to provide erratic indications when the liquid "sloshes" back and forth in the reservoir. There is also a need for a simple electronic monitoring circuit compatible with such a sensor.